The dimensions of a USB A plug should be 12mm wide by 4.5mm tall. You can use digital calipers to confirm that your socket will accept a plug of that size. If it doesn't, you've received a badly manufactured socket and should return the device.
USB A sockets are remarkably sensitive to being restricted by their housing.
In the above image I've circled the flexible tabs used on a common USB A socket. If the socket is inserted into a casing that restricts the movement of the tabs you'll find it incredibly difficult to jam a USB plug into it (as I found when designing wooden enclosures for USB power supplies). If the Pi is in a case, make sure that nothing is preventing these from flexing while you push in your plug. It's just about conceivable that they may have been accidentally bent in towards the centre of the socket opening, resulting in a higher required insertion force. You could try jimmying them open a little further using a small screwdriver. Be very careful not to break them!
The Pi 3 includes 2 separate USB socket modules, each of which houses 2 individual USB sockets, providing a total of 4 sockets. The chances of both USB socket modules being incorrectly manufactured are very very very small. Verify that the problem only exists in one of the dual-socket modules. If you can't fit any of your USB devices into any of the 4 sockets across the 2 modules then the problem is far more likely to be with your insertion method. Assess the way you're trying to plug your devices in and, if it's extravagantly weird, consider reducing any run-up, follow-through, gestural motions, radial arm-swinging, jiggling, shoogling, or unnecessary revolutions.